BALTIMORE -- The A's slide continued with Monday's 4-2 loss to the Baltimore Orioles in the opener of a three-game series at Camden Yards.

The Orioles scored all of their runs off Gio Gonzalez (5-4) in the first two innings.

The first was an unearned run, aided by shortstop Cliff Pennington's error. Then Baltimore strung together four hits off Gonzalez in their three-run second.

The A's have lost seven straight.

"We just haven't been playing well for the last week," A's manager Bob Geren said. "It's a total package. From the first pitch, and every single inning."

One bright spot was the season debut of third baseman Adam Rosales.

Activated from the disabled list after missing the whole season to this point with a fractured foot, Rosales hit a two-run homer in the fourth inning. He also made a diving backhanded stop of Vladimir Guerrero's sharp grounder in the fifth and made the long throw to get him at first.

"I like the energy he brings to the team, and obviously the skills he has also," Geren said. "The impact he made in his first game, although it was in a losing cause, that was our highlight."

Rosales said his foot feels good but hasn't completely regained its full strength yet. Therefore, he'll be on a two-day-on, one-off schedule for now.

Second baseman Mark Ellis left the game in the sixth after straining his right hamstring while pursuing a popup earlier in the game.

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Ellis said he would have a better idea of how he felt when he woke up Tuesday morning.

But CSNBA reported Monday night that Ellis would go on the disabled list and that second baseman Jemile Weeks, the A's first-round pick in 2008, would be called up from Sacramento for his major league debut.

Weeks was pulled early from the River Cats game at Colorado Springs on Monday night.

Geren said Guillermo Moscoso would start Tuesday's game and that lefty Josh Outman would pitch Wednesday. That means Moscoso will take the mound on just two days' rest after making relief appearances Friday and Saturday.

After going 6 for 12 in three games batting sixth, Daric Barton was back in the second spot Monday and went 1 for 4.